Amplifier biasing system



June 5, 1934. B. s. MOCUTCHEN 1,961,937

AMPLIFIER BIASING SYSTEM Filed vApril l, 1932 2 Sheets-Sheet l Almw u Dmm|u K l I juL June 5, 1934. B. s. McCUTCHEN 1,961,937

AMPLIFIER BIASING SYSTEM Filed April 1, 1932 2 Sheets-Sheet 2 L W J my]16 InvenZvr Patented June 5, 1934 UNITED ,STATES 7 1,961,937 AMPLIFIERBIASING SYSTEM BrunsonS. McCutchen, Princeton Township, Mercer County,N. J.

Application April 1, 1932, Serial No. 602,461

20 Claims.

My invention relates to systems for applying biasing potential to thegrid structure of one or more amplifier tubes, particularly in radioreceiving systems.

In accordance with my invention, the negative biasing potential appliedto the grid of an amplifier tube is derived from the voltage of thesignal itself, as received or as received and amplified, asdistinguished from a voltage of or derived from a local source ofenergy; and the grid potential is prevented from becoming excessivelynegative during reception of strong signals. More particularly, so longas the voltage of the signal impressed upon the amplifier input circuitdoes not exceed a predetermined magnitude, the grid of the tube is leftsubstantially free or isolated, in the sense there is no externalconductive connection therefrom to the cathode, to permit a condenser inthe grid cir- 20 cuit to accumulate negative charges flowing to the gridfrom the cathode during the positive swings or half waves of the signalvoltage, thereby to provide a negative biasing voltage, of or due to thesignal, affording faithful or good quality of reproduction of thesignal; but when, due to higher signal voltages, heavy crashes ofstatic, or the like, the biasing voltage becomes excessively high, aconductive discharge path'is temporarily provided to drain from the gridits excess negative charge to prevent distortion of reproduction orblocking of the tube; preferably, the discharge path includes aresistance of magnitude to ensure the rate of discharge shall not behigh enough to cause audible or other low frequency disturbances in areproducing device associated with the amplifier output system.

More specifically, a device having a critical or threshold breakdownvoltage, as a tube containing neon or equivalent, forms upon occurrenceof discharge therethrough a conductive path from grid to cathodewhenever the gridbiasing voltage tends to become excessively negative;alternatively, the anode of a thermionic tube, connected between gridand cathode of the amplifier, with its cathode presented to the grid ofthe amplifier, may be negatively biased so that a conducting path fromgrid to cathode external to the amplifier tube exists upon reception ofstrong signals to prevent excessive negative grid bias.

Further in accordance with my invention, the direct current component ofthe plate circuit current of the amplifier tube is prevented frombecoming excessively high when there is no signal or when the signal isof low magnitude; for

example, a ballast resistance may be included in the plate circuit tolimit the current rise; the plate current supply system may beconstructed or designed to have a voltage characteristic which isdecidedly drooping; or the increase in plate current may be utilized toincrease the gridbiasing voltage of the amplifier tube whenever it fallsbelow a predetermined minimum.

In one form of my invention, the discharge device, besides reducing asaforesaid the biasing voltage of the grid when built up to excessivelyhigh magnitude, as by strong signals, is also operative when the bias isless than a predetermined minimum, to permit recharge of the afore saidcondenser independently of the signal current until the desired minimumbias voltage is restored; more specifically, when the sum of the breakdown voltage of the discharge device and the grid-biasing voltage isless than the sum of a predetermined unidirectional voltage and thepositive peak voltage of an alternating current source, small currentimpulses flow through the device to the condenser, diminishing in amplietude until finally the biasing voltage has increased to such extent thatthe discharge de'- vice no longer to substantial extent conducts and theimpulses no longer flow.

My invention resides in the methods and apparatus hereinafter describedand claimed.

For an understanding of my invention and for illustrationof some of thevarious forms it may take, reference is to be had to the accompanyingdrawings in which:

Fig. 1 illustrates an audio frequency amplifier stage utilizing theinvention.

Fig. 1a is similar to Fig. 1 except that the amplifier is for radiofrequencies.

Fig.2 is a modification for maintaining the grid-biasing potentialbetween predetermined limiting values.

Fig. 3 illustrates a further modification of my invention, using athermionic tube as a discharge device.

Figs. 4, 4a and 5 illustrate further modifications of the invention.

Referring to Fig. 1, the grid g of the audio amplifier tube V isisolated from its cathode c by the blocking condenser K; accordinglythere is no conductive or direct current path except as hereinafterprovided, between the grid and cathode external to the tube. Whensignals of audio frequency are impressed upon the input circuit of thetube, through transformer T or otherwise, as by transfer from a prioramplifier stage, a detector, transmission line, phonograph pick-up, or

the like, the potential of the grid follows the signal voltage. Whenunder influence of the signal the grid is positive with respect to itscathode for at least part of the signal voltage cycle, electrons flowfrom the cathode to the grid within the tube, giving it a negativecharge. As the grid circuit is open for direct currents by virtue of theblocking condenser K, these charges do not leak off but are stored oraccumulated in the condenser K which so becomes a source of negativegrid biasing potential. The magnitude of capacity of condenser K is notcritical; it may be of from one to four microfarads, or even as low as.03 microfarad, more or less.

The grid is therefore free to accommodate itself to the strength of theincoming signal; the stronger the signal, the greater the chargeaccumulated by the condenser K and the greater is the negative gridbias. The biasing potential is therefore derived from or produced by thesignal voltage itself','and the amplifier automatically adjusts itselfto the proper operating condition.

If the signal voltage is of constant magnitude, and inherent leakageresistance is neglected, the grid assumes a potential more negative thanthe cathode by an amount corresponding to the peak value of the signalvoltage wave applied toit. Under such conditions the tube no longerdraws gridcurrent, the plate current is limited by the negative chargeon the grid, and in all respects the tube properly functions as anamplifier. If the signal voltage increases in amplitude, for example, sothat the grid again becomes positive for the positive peaks of thesignal waves, additional negative charges flow to the grid increasingthe condenser charge and .so increasing the negative biasing potentialuntil the tube again no longer draws grid current.

- However, under conditions of operation the signal-voltage impressedupon the grid may be far from uniform in amplitude, and in some cases itmay assume very excessive values for short periods of time, as forexample, during crashes of static in radio reception. With abnormallyhigh values of input voltage, the negative charge built up on the gridmay be so high as to cause serious distortion, and in extreme cases mayto such extent bias the grid negatively that the plate cur rent is cutoff, in which event the tube is said to be blocked. Thetube, onceblocked, remains inoperative until the excessive negative charge leaksoff the grid through very high resistance leakage paths inherent in thetube itself, the tube socket, wiring and the like. Until the excesscharge drains off, which may require a substantial period of time, theplate current does not flow and the tube is inoperative.

To insure continuous functioning of the tube and yet procure highquality of reproduction, a discharge device D is employed to preventblocking of the tube, or, in general, to prevent excessively high gridbiasing voltages. The discharge device may be one dependent uponionization of a gas, as neon, argon, mercury-vapor, etc., or it maydepend upon electro-chemical action, as in electrolytic condensers orrectifiers, or it may be electronic in nature.

In the arrangement specifically shown in Fig. 1, the discharge device isa neon glow tube of a well known type, specifically, it may be ahalfwatt type G10 manufactured by the General Electric Company, having abreak-down or threshold discharge voltage of about 120 volts. The tube Dis connected in a path between the grid g and cathode c which isnormally open because the potential difference between the electrodes ofthe tube D is less than the break-down voltage of the tube. When,however, the signal or input voltage is excessively high, as under theabnormal condition above mentioned, a conductive path is formed withinthe tube so that the excess negative charge is drained oif the gridpreventing the tube from blocking or causing distortion.

As the break-down potential of the tube D is generally substantiallyhigher than the signal voltages usually encountered in normal operationof the amplifier, the electrode of the discharge tube D presented to theamplifier cathode c is not directly connected thereto, but connects to apositive terminal 1 of a battery B, or equivalent, whose negativeterminal is connected to the cathode of tube V. The effect is the sameas if the break-down voltage of the tube D were in fact lower and morenearly equal to or comparable with the maximum permissible negative gridvoltage. While in this instance the source B is used for other purposesas well, it shall be widerstood a separate source, of suitable voltage,may be placed anywhere, as at X or Y, in series with the tube D betweengrid g and cathode c of tube V.

Or if the tube D should have a break-down voltage substantially lessthan the maximum de sired negative grid-biasing voltage, a source ofuni-directional voltage, such as B, may again be used in like positions,but poled reversely to the foregoing cases.

Using a UX245 tube as an audio amplifier, and a plate potential of 250volts, it was found that when the negative grid bias exceeded 60 volts,distortion resulted. For this case, the tap 1 from the discharge tube Dto source B should be connected to a point or terminal of the latterwhich is about 60 volts positive with respect to the cathode 0. Withthis connection, the grid circuit remains open and the grid is free toassume any negative potential up to 60 volts in accordance with thestrength of the impressed signal. When, however, the positive peaks ofthe signal or input waves exceed 60 volts, the potential across the tubeD is greater than its break-down voltage of 120 volts, the tube Dconducts, and the excess negative grid charge is drained off.

To prevent the rapidity of action of tube D and its circuit fromcreating an audio frequency disturbance, a resistance 2, of relativelyhigh magnitude, as of the order of l megohm, is connected in series withthe discharge device D. When the tube breaks down, the resistance 2prevents such disturbance and besides limits the amplifier grid currentto a very small magnitude so that the distortion is not serious.However, if the signal or input voltage is so strong that the tube Dcontinuously flashes, the signal lever should be reduced in any suitablemanner, as by a usual volume control which reduces the intensity of thesignal voltage impressed upon the input of tube V.

Under conditions of no signal, or of reception of weak signals, thenegative grid charge, though tube D is then not conducting, graduallyleaks off from condenser K, and the grid of the amplifier therefore veryslowly approaches cathode potential. Particularly if the tube V is ofthe so-called power type, and the voltage of the plate current source Bis of proper value for normal operation of the tube, the plate currentunder the conditions of no or weak signals, or low or no negative gridbias, will tend to be excessive,

with likelihood of damage to the tube and of abnormal current drain onthe source of anode current B. In the system shown in Fig. 1, such unduerise of plate current is prevented by the ballast resistance 3 ofsuitable magnitude, and preferably of material having a high positiveresistance-temperature coefficient. With the grid 9 approaching or atcathode potential, the increase in anode current through resistor 3reduces the difference in potential between the anode or plate a of thetube and its cathode c, with reduction of plate current through the tubetosafe values. As shown, source B and resistor 3 may be shunted by anaudio-frequency by pass condenser K1. 1

Although transformers T and T1 have been shown for coupling the inputand output circuits of the tube V respectively, to the immediatelypreceding and followingcircuits or devices, it is of courscunderstoodthat any other form or" coupling device, such as resistance,auto-transformer, etc, may be used. 1

The invention is not limited to use with audioirequeney amplifiers, for,as shown in. Fig. la, it may also be used with high or radio-irequencyamplifying systems.

Fig. 1a is in general identical with Fig. 1, eX- cent the couplingtransformers T2, T3 are of the radio-frequency type. When the inputcircuit of the amplifier tube is to be tuned, as by a con denser C, vriable'or not, the blocking condenser K, the discharge device D, and itsappurtenances, are preferably outside of the tunable loop formed by thesecondary S of the transformer and the tuning condenser C. The inputcircuit of the tube V may be coupled to an antenna, to a precedingamplifier stage, to a carrier frequency transmission system, or thelike, and the output circuit may be coupled to another amplifier, to theinput of a detector, etc. The magnitude of capacity K may be lower thanfor the audio frequency system of Fig. 1, although again that magnitudeis not critical, and may, for example, be of the order of .03microfarad.

The system shown in Fig. 2 is generally similar to the system of Fig. 1,differing principally in that it utilizes a different arrangement forlimiting the anode current under conditions of no signal or low signalvoltage. In this modification, an impedance 4 of suitably highresistance is connected between the negative terminal of the battery B,or equivalent, and the cathode c or" the tube, so that the point 5 ismore negative than the cathode due to the flow of anode current throughresistance The magnitude of resistance l is so chosen that when theplate cur-- rent flowing through it is somewhat less than thepermissible maximum for the tube, the drop of potential across theresistance exceeds the breakdown voltage of a second discharge deviceD1, so that there is a surge which charges condenser K again to providea negative grid-biasing potential. This action may repeat from time totime as the grid charge slowly leaks off, or until a sufficiently strongsignal is impressed upon the input circuit of the tube which thereuponoperates in the normal manner described in connection with tube D ofFig. 1. The second discharge device D1 may be of the same characterdevice D, preferably there is included .1 series with D1 a suitably highresistance 6. This anode current limiting arrangement may in like way beapplied to the radio frequency amplifier system of Fig. la, or in any ofthe other systems herein described.

In the system shown in Fig. 3, the condenser is. is connected betweenthe grid and the hi h-potential side of the transformer secondary S1,but insofar as the operation of either this arrangement or thearrangement shown in Fig. l is concerned, the condenser K may be eitherin the grid lead or cathode lead. For normal amplitudes of signalvoltage, the system operates as described in connection with Fig. 1, thecondenser K storing the negative charges flowing to the grid to providea negative grid biasing potential. In this modification, however, anelectronic tube, as a two element thermionic tube or valve 132,preferably of very high vacuum or substantially pure electron discharge,is used instead of an ionization discharge tube D or the like. Thecathode h oi the tube D2 is presented to the grid 9 of the amplifiertube V. The anode p of tube D2 is connected through battery B1 andresistance 2 to the cathode c of the amplifier tube V. The battery B1has its negative pole connected to the anode p of the tube D2, thisbeing contrary to the usual practice for an anode battery. The voltageof battery B1 is chosen equal to the maximum negative bias which is tobe permitted for the grid g of the amplifier tube V. No electrons canflow from cathode to anode of tube D2 until the cathode becomes morenegative in potential than the anode, which is to say until the inputsignal voltage has built up a negative charge on grid g of amplifiertube V which is greater than the potential of battery B1. When such acharge has been built up tube D2 becomes a unidirectional conductor andthe excess charge on grid g is drained off. Resistance 2 limits the rateof discharge through tube D2 and thus prevents sudden disturbances inthe output of the amplifier.

The tube D2 may be of the uni-potential cathode type, as an UXZZ'I, inwhich event the source of heater current Al, Whether a battery or atransformer winding, need not be at high potential with respect to thecathode of the amplifier tube V.

Fig. 4 illustrates the invention as embodied in a resistance coupledamplifier system, and in which the anode current is supplied by arectifier-filter network FR. The condenser K for providing the negativepotential for the grid of amplifier tube V may be connected between thegrid of tube V the anode of the preceding tube V1, so that it alsoserves as a coupling condenser between the tubes. Under thesecircumstances it should e suitably large to afford low impedance toaudio frequencies; for example, it should be of the order of .1microfarad or larger.

Particularly when the preceding tube V1 is functioning as a detector aradio-frequency choke coil '7 is preferably connected between thecondenser K and grid 9' of the amplifier tube V, a radio-frequencyby-pass condenser (32 connec ed between the anode and cathode of thedetector tube. The usual coupling resistance 8 is included in the anodecircuit of the tube V1, the condenser 9 affording a path of lowimpedance to ground or cathode in shunt to the power supply. The directcurrent component of the anode current of tube Vl through the conductor19 which may, as shown, be connected to a tap on the resistance 11 inthe output of a filter rectifier system FR.

The usual grid leak between the grid g and the cathode c of theamplifier tube V is omitted, so that the negative charges flowing to thegrid for positive swings or half waves of the signal voltage are storedby the condenser K to negatively bias the grid g of the amplifier tube.As in Fig. 1,

the discharge device D, in series with a suitably high resistance 2, isconnected between the grid of the amplifier tube and its cathode. As inFig. 1, if the break-down voltage of the discharge device D is higherthan the permissible maximum swing of the signal voltage, the device D,instead of being connected directly to cathode, connects to a point ortap la which is suitably positive with respect to cathode; for example,to a tap or slider on the bleeder resistance 11 in the power supplysystem FR, or more generally, to any point in the anode supply systemwhich is to proper extent positive with respect to the amplifiercathode. When the impressed signal is so great that the sum of thevoltage built up on condenser K by the signal, and the voltage frompoint 1a to cathode is greater than the breakdown voltage of thedischarge device D, a leakage path is formed between the grid andcathode through device I) to remove the excess of negative grid charge.

Under conditions of no or low signal, the plate current tends toincrease. To prevent the anode current rise from becoming excessive, thefilter rectifier system FR may be designed or chosen to have a droopingvoltage-load characteristic; that is, the supply transformer, therectifier tube R, and/or the conductive impedances I of the filter F,may have such characteristic that the plate current of tube V is limitedto safe values even though the grid g should assume cathode potential.Alternatively, or in addition, a ballast resistor 3 may be included inthe plate circuit of tube V for this purpose, as described in connectionwith Fig. 1.

While it is more convenient to obtain the supplemental voltage fordischarge tube D from the power supply system, a separate battery B2 maybe used, as shown in in; the positive terminal of the battery B2 ispresented to the grid g of the amplifier tube V, but the path isnormally open because of the intervention of the discharge device D. Asin the preceding figures, the grid circuit is normally open, but uponoccurrence of excessively strong signals or input voltage, theconductive path from grid to cathode is formed through tuoe D, to drainoiT or neutralize the excess negative rid charge.

In the system shown in Fig. 5, the condenser K, as in the precedingmodifications, provides a biasing potential from the signal or otherinput voltage for grid-biasing purposes. In addition, in this case, thetap or contact ii) is connected to a potentiometer resistance across thewinding W of the supply transformer for the tube V, or instead it mayconnect to a tap on the winding W itself. In either event, the voltagebetween the point So of the filter network and contact lb is analternating current voltage; it is in series with the direct currentvoltage drop across the impedance or resistance 12 in the negativeconductor of the filter F so that the point 5c is negative with respectto cathode c, and the point lb is alternately positive and negative withrespect to cathode c.

For simplicity of explanation, it is assumed that the breakdown voltageof the discharge device D is 120 volts and the desired minimum negativebiasing potential for grid 9 is 30 volts; it is also assumed that undernormal conditions the voltage drop across the filter reactor 12 is 40volts, and that the contact 12) has been adjusted so that the peakalternating current voltage between it and point 50. is 110 volts. Underconditions of normal signal strength, the condenser K stores thenegative charges, due to the signal alone, flowing to the grid for gridbiasing purposes. However, under conditions of no signal or low signalvoltage, as previously pointed out, the grid potential gradually driftstoward that of the cathode. The grid is also substantially at cathodepotential if the amplifier is put into operation after a fairlyprotracted period of non-use. Under these circumstances, the tube D issubjected to a difference of potential equal to the algebraic sum of theso volt drop across the filter reactor 12, and the alternating potentialdifference of 110 volts between contact 1b and point 511.. For thepositive half waves of the power cycle, the sum of these voltages is 70volts, which is less than the break-down or critical voltage of the tubeD. For the negative half waves of the power cycle, however, the sum is150 volts, which is sufficient to break down the tube D, and to permitthe condenser K to acquire a charge. As soon as the charge is built upto a value of 30 volts negative on its plate presented to the grid, thetube D no longer breaks down, for the sum of its breakdown voltage andthe biasing voltage is not less than the sum of the filter reactor dropand the negative voltages picked oil by contact lb.

If the charge leaks off after a time, the tube D again breaks down andthe cycle repeats until the desired minimum of 30 volts negative gridbias is reestablished.

If a signal having a peak voltage in excess of 36 volts is impressedupon the amplifier input circuit, the grid becomes more and morenegative for stronger and stronger signals as in the priormodifications. When very large signal or other voltages are impressed onthe amplifier input, the grid becomes more and more negative, untilunder the conditions above assumed, it reaches a negative potential of50 volts. When this condition is exceeded, the tube D will again breakdown, the excess negative grid charge will be drained off. The tubebreaks down because the sum of the bias voltage built up on condenser Kand the algebraic sum of the aforesaid voltages in the power supplysystem for the posive halves of the power cycles is in excess of 120volts.

Briefly, in this arrangement, for the assumed values, the grid ipermitted to float freely and to adjust itself to signal voltagesbetween the limits of 30 and 50 volts. If the upper limit of biasingvoltage, 50 volts, is exceeded, the excess charge at once leaks ofithrough the path afiorded by break-down of the tube D. If signalVoltages less than the ininimum, 30 volts, are applied for long enoughtime to allow the charge on the condenser K to leak off, through pathsinherent in the condenser, tube, wiring, and appurtenances, to suchextent that the condenser charge falls below the minimum of 30 volts, anew charge is applied to condenser K from the power supply system aspreviously described until the minimum of 3.0 volts is againestablished. It is understood, or course, that the foregoing numericalvalues are used for illustrative purposes, and not in a limiting sense.If, for example, the contact lb set to pick off a peak alternatingcurrent voltage or" 115 volts instead of 110 volts, the limits of freegrid operation is then from 35 volts negative to 45 volts negative.

In the foregoing description, the secondary effect of the grid bias uponthe amplifier plate current which in turn affects the voltage dropacross the filter reactor 12 has been omitted for simplicity ofexplanation. Actually, this secondary efiect tends to narrow the rangeof free grid operation for a given setting of the potentiometer contactlb.

The voltage drop across the filter reactor, or more generally betweenthe point 5a and cathode 0, should equal the normal or optimum biasingvoltage of the tube, 1. e., if the tube is of the (1X24? pentode typeusually operating with a negative grid bias of 16 volts, the voltagedrop across the reactor 12 should. be 16 volts. If the total voltagedrop across the reactor is greater than that amount, the desired valuecan be obtained by using a potentiometer resistance shunting the reactoror by tapping the reactor. On the other hand, if the drop across thereactor is not sufiicient, the desired voltage may be obtained by takingthe voltage drop across the reactor and a resistance in series therewithbetween cathode c and point 5a.

For brevity in the appended claims the term signal voltage is utilizedgenerically to include all input voltages of or from external sources asdistinguished from local sources of current or power supplies; and thesignal voltage may be 25. that of or representing any type oftransmission,

at either radio, audio or other frequency, includ ing telegraphy,telephony, television and the like.

What I claim is: 1. The method of biasing the grid of an amplifier,which comprises impressing a signal voltage upon the grid circuit of theamplifier, accumulating negative charges flowing to the grid under theinfluence of said signal voltage to effect a negative grid biasingpotential, applying said negative potential to an electrode of adischarge device having another electrode connected to the cathode ofsaid amplifier, and separately, producing an opposing potentialdifference between said. electrodes whose magnitude is selected to beara predetermined relation to the maximum desirable negative grid-biasingpotential so that when the said biasing voltage exceeds a predeterminedmagnitude, the conductivity between said electrodes, external to thetube and between grid and cathode thereof, is increased to reduce saidbiasing voltage.

2. The method of biasing the grid of an amplifier, which comprisesimpressing a signal voltage upon the grid circuit of the amplifier,accumulating negative charges flowing to the grid under the influence ofsaid signal voltageto effect a negative grid biasing potential, applyingsaid nege ative potential to an electrode of a discharge device havinganother electrode connected to the cathode of said amplifier, andseparately producing an opposing potential difference between saidelectrodes whose magnitude is selected to bear a predetermined relationto the maximum desirable negative grid-biasing potential so that onlywhen said biasing voltage exceeds a predetermined magnitude,conductivity is established, external to the tube, between grid andcathode thereof, to reduce said biasing voltage.

3. The method of biasing the grid of an amplifier, which comprisesimpressing a signal voltage upon the grid circuit of the amplifier,accumulating negative charges flowing to the grid under the influence ofsaid signal voltage to effect a negative grid biasing potential,applying said negative potential to an electrode of a gaseous dischargedevice having another electrode connected to the cathode of saidamplifier, and applying to said electrodes an opposing potentialdifference whose magnitude is selected to be substantially equal to thedifierence between the ionizing potential of the gas and the maximumdesirable negative grid-bias so' that when said bias- 7 ing voltageexceeds a predetermined magnitude, the excessive biasing voltage ionizesthe gas to conduct the excessive charge from the grid and so reduce saidbiasing voltage.

4. The method of biasing the grid of an amplifier, which comprisesimpressing a signal voltage upon the grid circuit of the amplifier,accumulating negative charges fiowing to the grid under the influence ofsaid signal voltage to efiect a negative grid biasing potential,applying said negative potential to an electrode of an electronic devicehaving another electrode connected to the cathode of said amplifier, andapplying to said electrodes an opposing potential difference selected tobe of such magnitude as to prevent electronic conduction between saidelectrodes except when the negative biasing potential. becomesundesirably high.

5. The method oi" biasing the grid of an amplifier which comprisesleaving the grid circuit open as to unidirectional current, impressing asignal voltage upon the grid circuit, accumulating the negative chargesflowing to the grid from cathode to provide a negative grid biasingpotential, applying said negative potential to an electrode of adischarge device having another electrode connected to the cathode ofsaid amplifier, and applying an opposing potential difference to saidelectrodes selected to be of such magnitude that under conditions ofexcessively high signal voltage, the grid circuit is intermittentlycompleted to drain off the excess grid charge, and selecting theresistance of the drainage path to be of such high magnitude that thedrainage is at sub-audible rate to avoid disturbing low frequencyvariations of the anode current of the amplifier.

6. In the operation of a thermionic amplifier, the method whichcomprises impressing a signal voltage upon the grid circuit of theamplifier, accumulating negative charges flowing to the grid to 'efiectnegative grid biasing potential, and limiting the anode current of theamplifier under conditions of low signal voltage or of no signal.

'I. In the operation of a thermionic amplifier,

the method which comprises impressing a signal voltage upon the inputcircuit of the amplifier, accumulating the negative charges flowing tothe amplifier grid to provide a biasing potential therefor, and, underconditions of no signal or low signal voltage, increasing the impedanceof the anode circuit of the amplifier to limit its anode current.

8. In the operation of a thermionicamplifier, the method which comprisesimpressing a signal voltage upon the input circuit of the amplifier,accumulating the negative charges flowing to the amplifier grid toprovide a biasing potential therefor, temporarily providing a leakagepath to drain off part of the grid charge when the biasing potential isexcessive, and, when the biasing potential is low, temporarilyaccumulating a charge derived independently of signal voltage, torestore the biasing potential.

9. In the operation of a thermionic amplifier, the method whichcomprises impressing a signal voltage upon its input circuit,accumulating the negative charges flowing to the amplifier grid toprovide a biasing potential therefor, temporarily providing a leakagepath when the biasing potential is excessive, and, under conditions ofno or low signal voltage producing by the resultant increase in platecurrent a voltage to restore the grid biasing potential to magnitude forwhich the plate current is not excessive.

10. An amplifier system comprising a thermionic tube, an input circuittherefor open as to unidirectional current and including in series acondenser for storing the negative charges fiowing under the influenceof the signal voltage to the grid from cathode to derive a negativebiasing potential for the grid, and a discharge device for closing saidgrid circuit for unidirectional current during reception of strongsignals thereby to prevent distortion.

11. An amplifier system comprising a thermionic tube, an input systemtherefor open as to unidirectional current and including a condenser forstoring the negative charges flowing to the grid from cathode to derivea negative grid biasing potential from the impressed signal, a dischargedevice between grid and cathode of said tube for closing said gridcircuit for unidirectional current upon reception of strong signals todrain'ofi the excess grid charge otherwise causing distortion, and ahigh resistance in series with said discharge device to ensure that therate of discharge shall be at a frequency other than audible.

12. An amplifier system comprising a thermionic tube, an input circuittherefor open as to unidirectional current and including a condenser forstoring the negative charges flowing to the grid from cathode to derivea negative grid biasing potential from the impressed signal, a normallyopen path in shunt to said condenser, a discharge device in said path,and a source of di rect current in said path having its negative polepresented to the cathode of said tube and its positive pole to the gridof said tube, the sum of the voltage of said source and of the negativebiasing voltage built up on said condenser by said strong signals beinggreater than the breakdown voltage of said device.

13. An amplifier system comprising a thermionic tube, a blockingcondenser between the grid and cathode of said tube for accumulating thenegative charges flowing to the grid from cathode to derive a negativegrid biasing potential from the signal, a unidirectional current systemfor supplying the anode current of said tube, and a normally open directcurrent path from grid to cathode including a discharge device connectedbetween the grid and to a point in said anode supply system whosepotential is substantially positive with respect to cathode.

14. An amplifier system comprising a thermionic tube, an input circuittherefor including a condenser between grid and cathode for deriving anegative grid biasing potential from the impressed signal, a dischargedevice forming a normally open direct current path in shunt to saidcondenser and intermittently closing said path during reception ofstrong signals to prevent distortion by excessive biasing potential, andmeans for limiting the plate current of the tube during conditions forwhich the grid approaches the cathode in potential.

15. An amplifier system comprising a ther mionic tube, a condenser inthe input circuit of said tube for deriving negative grid biasingpotential from the impressed signal, a source of direct current havingits negative terminal connected to cathode, and a glow tube connectedbetween the grid and a positive terminal of said source.

16. An amplifier system comprising a thermionic tube, a condenser in theinput circuit of said tube for deriving negative grid biasing potentialfrom the impressed signal, a system for supplying the anode current ofsaid tube, and a glow tube connected between the grid and a point insaid system more positive than the cathode of said thermionic tube.

17. An amplifier system comprising a thermionic tube, a condenser in theinput circuit of said tube for deriving a negative grid biasingpotential from the impressed signal, a normally open path between thegrid and cathode of said tube, a thermionic tube in said path having itscathode connected to the grid of said first tube and its anode to thecathode of said first tube, and a source of voltage in said pathnegatively iasing the anode of said second tube with respect to itscathode.

18. An amplifier system comprising thermionic tubes, a blockingcondenser coupling the anode circuit of one tube to the input circuit ofthe next tube and deriving negative grid biasing potential from theimpressed signal for the grid of said second tube, a system forsupplying the anode current of said tubes, and a normally open pathcomprising a discharge device connected between the grid of said secondtube and a point in said supply system more positive than the cathode ofsaid second tube.

19. An amplifier system comprising thermionic tubes, a blockingcondenser coupling the anode circuit of one tube to the input of thenext and deriving negative grid biasing potential from the impressedsignal for the grid of said second tube, a system for supplying theanode current of said tubes having a falling voltage-loadcharacteristic, a glow tube connected between the grid of said secondtube and a point in said supply system more positive than cathodepotential, and a high resistance in series with said glow tube.

20. An amplifier system comprising a thermionic tube, a blockingcondenser in the input circuit thereof between grid and cathode, arectiher-filter system for supplying the anode current of said tube, anda discharge device connected between the grid of said tube and a pointin said system between which and cathode there are two sources ofelectromotive force in series, one a source of unidirectional force withthe positive terminal presented to cathode, and the other a source ofalternating electromotive force alternately positive and negative withrespect to cathode.

BRUNSON S. MCCUTCHEN.

